1. Field of the Invention
The present invention relates to a self-dispersible aqueous pigment dispersion having easy dispersibility in an aqueous system and excellent dispersion stability over time, and a method for preparing the same. Further, the present invention relates to an inkjet recording liquid produced from the aqueous pigment dispersion, which provides excellent light-resistance, water-resistance and hue of printed image, and simultaneously reveals excellent discharge stability at the nozzles of ink-jet printer.
2. Description of the Prior Art
Conventionally, aqueous ink and aqueous paints have been prepared using a surfactant, dispersing agent, or dispersing resin for dispersing a pigment in water. In the case of a surfactant, a foaming problem exists, and co-use of a de-foaming agent is necessary for improving foaming property. However, the co-use of the surfactant and other additives leads to problems in balance of ink properties, such as compatibility of the de-foaming agent and the surfactant, and a leveling problem in drying after printing. In the case of a dispersing resin, extremely effective dispersion stabilizing effect is recognized owing to the resin, in the ink or paint of relatively high viscosity. However, in the case of dispersion having relatively low viscosity, dissolution of the resin into the solvent imparts a significant influence on the viscosity of the dispersion, giving limitation to lowness of viscosity.
On the other hand, there have been conventionally suggested methods in which the surface of a pigment powder is chemically modified for achieving sufficient dispersion property of the pigment. As industrially effected examples, surface treatments to carbon black utilizing gas phase or liquid phase process are found. The gas phase method aims at manifestation of a polar functional group by oxidation treatment such as ozone treatment and plasma treatment. The liquid phase method is used when further intense treatment is required than in the oxidation treatment usually conducted by the gas phase method, and uses a stronger oxidation agent such as nitric acid, sodium nitrite, sodium hypochlorite, permanganate, and the like. By conducting these treatments, dispersing a pigment into an aqueous system is possible without using a dispersing agent and resin. Their application range has been enlarged, therefore. However, in these cases, a demerit of high production cost is present due to features of the treating processes, and further, application of the technology to an organic pigment is impossible, resulting in limitation of use range.
Further, there have been known methods in which a pigment derivative is specifically used as a dispersing agent. Pigment particles are surface treated by the derivative in these methods. Namely, the pigment derivative treating method is a method in which an acidic group or basic group is introduced as a substituent into a compound of the same kind as a pigment to be dispersed, to give a pigment derivative, which is allowed to be adsorbed onto the pigment particles surface to increase its polarity, to improve its wetting property, and therefore, to enhance the dispersibility of pigment particles. However, these methods have problems that bleeding and color mixing tend to occur when the paint or ink produced therefrom is actually applied onto matter for painting or printing. Also, these methods have such a fatal problem unsolved, namely thixotropic characteristics of the system, that dispersed particles may easily happen to flocculate even if the dispersion of particles has been once attained. This has made it difficult for us to achieve a stable dispersion of pigment particles only by using a pigment derivative. Various trials have been made on addition of a surfactant, organic solvent and other additives to this dispersion system to improve the property, however, there have been actually no basic solution found. Rather, these additional additives make the disperse system complex and results in the troubles such as dewetting (crawling) during drying a coated film, etc.
Regarding their applications, recording liquid for inkjet printing, which has so proliferated in recent years, is one example for practical use of such aqueous coloring materials prepared by dispersing a dye or pigment in an aqueous system. As inkjet recording liquid, one which is prepared by dissolving a water-soluble dye in an aqueous solution composed of a glycol-based solvent and water has been conventionally used. However, the resulting recorded article has a problem of poor water-resistance since the recording liquid contains a water-soluble dye, and for improving this problem, a recording liquid prepared by dispersing an organic pigment in an aqueous medium has been developed. However, it is extremely difficult, as described above, to disperse a pigment in the form of a minute particle in an aqueous system and maintain the dispersed condition stably, unlike a dye which is dissolved in an aqueous system. In addition, high resolution of recent inkjet printing has made the nozzle diameter extremely minute. Accordingly, the particle size of a pigment used in the recording liquid is also required to be made finer, consequently, high dispersion of minute particles, which is a higher dispersion technology than that of the conventional ones, is needed. Further, properties naturally required for a recording liquid such as discharge stability from printer nozzles, re-dispersibility, color developing property after printing, and the like should also be maintained together, and under actual condition, simultaneous satisfaction of the properties required for an inkjet recording liquid has not been accomplished.
Accordingly, the primary object of the present invention is to provide a self-dispersible aqueous pigment dispersion having excellent storage stability over time. Another object of the present invention is to provide a pigment type inkjet recording liquid which manifests discharge stability at nozzles, and has sufficient color-reproducing range when printed, and has excellent water-resistance and light-resistance without bleeding and color mixing as above-mentioned.
The present invention disclosed herein is a self-dispersible aqueous pigment dispersion comprising an aqueous solution, particles of a pigment and a water-soluble pigment derivative, said particles being dispersed in the aqueous pigment dispersion in a concentration within the range from 2 to 25 wt. % based on the amount of the aqueous pigment dispersion, wherein a part of the water-soluble pigment derivative is adsorbed on the surface of the particles of the pigment to form dispersed particles comprising the particles of the pigment adsorbed by said part of the pigment derivative, and the remaining parts of the water-soluble pigment derivative are free in the dispersion without being adsorbed on the surface of the pigment in a concentration less than the critical flocculation concentration on which flocculation is initiated on said particles of the pigment dispersed under said concentration.
The water-soluble pigment derivative preferably comprises a compound in which an acidic functional group or salt thereof has been introduced into the skeleton of the pigment molecule. In this case, the acidic functional group is preferably a carboxylate group or sulfonate group.
In the pigment dispersion of the present invention, the dispersed particles preferably have an average particle diameter of 10 to 300 nm. Furthermore in the present invention, it is preferable that the particles of the pigment are dispersed in the aqueous pigment dispersion in a concentration within the range from 15 to 20 wt. % based on the amount of the aqueous pigment dispersion, and the viscosity of the aqueous pigment dispersion is 3 to 20 cps.
Regarding the pigment derivative, according to the present invention, said part of the aqueous pigment derivative adsorbed on the surface of the pigment is preferably in an amount ranging from 0.01 to 1.5 mmol/g based on the amount of the particles of the pigment.
Regarding pigment type, it is preferable that the pigment comprises a condensed polycyclic pigment. The condensed polycyclic pigment may be a quinacridone pigment or phthalocyanine pigment.
To accomplish the objects of the present invention, there is also disclosed herein a method for preparing a self-dispersible aqueous pigment dispersion in which particles of the pigment are dispersed in a concentration within the range from 2 to 25 wt. % based on the amount of the aqueous pigment dispersion, said method comprising the steps of (a) dispersing said particles of the pigment in an aqueous solution in which a water-soluble pigment derivative has been dissolved, to allow a part of the pigment derivative to be adsorbed on the surface of the particles, thereby forming dispersed particles comprising the particles of the pigment adsorbed by said part of the pigment derivative, and (b) removing the remaining parts of the pigment derivative free in the aqueous solution without being adsorbed in the step (a) to a concentration less than the critical flocculation concentration on which flocculation is capable of being initiated on said particles of the pigment dispersed under said concentration. In the method, the step (b) may comprise ultrafiltration.
In the method, the water-soluble pigment derivative preferably comprises a compound in which an acidic functional group or salt thereof has been introduced into the skeleton of the pigment molecule. The acidic functional group is preferably carboxylate residue.
In the method, it is desirable that the dispersed particles have an average particle diameter of 10 to 300 nm. It is preferable that the particles of the pigment are dispersed in the aqueous pigment dispersion in a concentration within the range from 15 to 20 wt. % based on the amount of the aqueous pigment dispersion, and the viscosity of the aqueous pigment dispersion is 3 to 20 cps.
Tthe step (a) is preferably conducted until said part of the pigment derivative adsorbed on the surface reaches 0.01 to 1.5 mmol/g based on the amount of the particles of the pigment.
Regarding pigment type used in the method, the pigment preferably comprises a condensed polycyclic pigment. The condensed polycyclic pigment may be a phthalocyanine pigment or quinacridone pigment.
The present invention also includes an inkjet recording liquid comprising the aqueous pigment dispersion heretofore described, a surfactant and/or an aqueous resin.
The present inventors have recognized that due to the substituent groups the pigment derivatives have some solubility to solvents, especially that of aqueous type, to which pigment particles must be dispersed, and have suspected that interactions between the solvent system and derivative molecules dissolved therein might causes, at least as one of the factors, the thixotropic unstableness as mentioned above, as well as bleeding and color-mixing when printed. In a practical routine, when pigment particles is intended to be dispersed in an aqueous solvent by adding a dispersing agent such as pigment derivative or some surfactant, it has been typical that, taking into account the adsorption/desorption equilibrium, they are added in amounts larger than the amount to be adsorbed on pigment particles, with the intention of making their dispersing performance to be at sufficient level.
The present invention has been accomplished based on the inventors"" following points: (1) the additional amount required to achieve a sufficient level of dispersing is required at least on the stage of dispersion process in which the adsorption is forced to occur, not necessarily might be required after the adsorption has been once accomplished; (2) the uncertainty of how strongly and/or stably the adsorption can be attained actually, opposed to the directiveness to the equilibrium of adsorption/desorption in a derivative-diluted dispersion system; and, (3) the actual uncertainty of how effective the desorbed-derivative-deprived condition of the dispersion system is for its stability and accompanying property such as non-bleeding, non-color mixing of printed image. As detailed hereinafter and non-limitatively illustrated in the examples, the present inventors have achieved the method and product as those of practically useful degree based on those above conceptions and deliberate investigations so far conducted.
The aqueous pigment dispersion of the present invention shows, when used in an inkjet recording liquid, excellent water-resistance as printed, and good stability of the dispersed state as stored, causes no clogging at nozzles when charged in printers, and provides stable discharge over a long period of time during printing. The aqueous pigment dispersion shows, when printed on paper, sufficient color density as one of print qualities, a wider color reproducing range, and more excellent light-resistance as compared with a dye type dispersion. Therefore, the aqueous pigment dispersion of the present invention can be utilized in wider range as a colored print in fields such as document making, address writing of mails and, marking, numbering and bar cord imparting of corrugated boards in offices. In short, when the present invention is applied to an inkjet recording liquid of aqueous pigment dispersion type, which is desired to be improved recently as above described, the utility thereof is thought to be particularly large. However, the present invention is not necessarily limitative to inkjet printing, the dispersion can also be used in gravure ink, aqueous paint and other printing ink fields.